This invention relates to a method of removing aldehyde impurities from .alpha.,.beta.-unsaturated carboxylic acid esters, such as acrylate esters and methacrylate esters (hereinafter "(meth)acrylate esters"). Processes for producing such esters which incorporate oxidative steps, such as the vapor phase oxidation of propylene, isobutylene, tertiary butanol, methacrolein, acrolein, or isobutyraldehyde to afford acrylic or methacrylic acid (hereinafter (meth)acrylic acid) followed by esterification to a (meth)acrylate ester, are known to result in product mixtures which contain quantities of aldehyde and other carbonyl containing impurities, such as benzaldehyde, furfural, protoanemonin, methacrolein, and acrolein. These impurities are undesirable because they may react with the (meth)acrylate esters in subsequent reactions, they may interact with other reactants which are intended to react with the (meth)acrylate esters in subsequent reactions, they may react to form colored impurities, or they may directly inhibit subsequent reactions. In addition, these materials may interfere with subsequent purification of the (meth)acrylate, whether at the acid or ester stage. For these reasons it is highly desirable to remove these impurities from the (meth)acrylate esters.
The use of bisulfites for removal of aldehydes and protoanemonin from methacrylic and acrylic acids is known. See for example, Japanese Patent No. 62-45222B and European Patent No. 0 102 642 A1 which disclose processes for purifying methacrylic acid with an aqueous bisulfite solution. These processes require several distillation steps to remove high and low boiling materials. In addition, these methods all require aqueous solutions of both the acrylic and methacrylic acid and the bisulfite with the resulting disposal and recycling problems. Thus, it is known that bisulfites are useful for removing carbonyl containing impurities, especially aldehydes and protoanemonin, from (meth)acrylic acids. The bisulfite acts by reacting with the carbonyl group to form the corresponding sulfonate ester salt. Such salts are water soluble and are easily separated from (meth)acrylic acid by use of solvent/solvent extraction using a water immiscible organic solvent. However, these methods all require the use of water-immiscible solvents to allow extraction of the carbonyl containing impurities into an aqueous phase. The aqueous phase is then separated from the acid. Solvent removal is then necessary for further purification.
We have discovered that carbonyl containing impurities can be removed from esters of .alpha.,.beta.-unsaturated carboxylic acids by extraction with bisulfite under neutral conditions rather than under the previously known acid conditions. As a result, treating esters on an industrial scale may be more cost effective than treating acids. Advantages of this invention are that the use of solvents is reduced or eliminated and unconsumed bisulfite may be easily recycled. Elimination of the solvent simplifies the purification process and reduces undesirable organic waste generation.